We are now embarking on what is turning out to be an interesting and enlightening project. We’re building a Passiv Haus certified bungalow (although 40% of the house has a basement). This may not sound like much but as I am finding out, Passiv Haus works much better on two storey buildings as the surface area to volume ratio is far more favourable.
Some people think Passiv Haus is a very extreme level of construction (in terms of thermal performance and detailing) and not really necessary. Whilst on some levels this may be true for mass housing, the benefits of going through the process for a bespoke project are numerous. Because of the level of detail required by PHPP (Passiv Haus Planning Package) it encourages the designer to ensure that virtually every element of the build is designed and thoroughly considered prior to the project going to site. This is very important for the person paying the bills as it enables the contractors involved to price very accurately.
To get our building to achieve the estimated 15kWh or less required by PH we have had to bring the U-values of the walls down to just under 0.1 W/m2K. This has resulted in walls just over 500mm thick in total, which actually suits us as we have lived in a Cob house for the last 11 years and so we are used to very thick walls.
The original idea was to use Tradical Hemcrete with a layer of Unger-Diffutherm wood fibre insulation on the outer face to give the building a good level of thermal mass. However, as the design was progressing WARM Associates (our Passiv Haus assessors) were indicating the likely U-values would be below 0.1 W/m2K meaning that the initial design would end up being around 700mm thick and costly too. In the end we settled on using 300mm of UdiFLEX flexible wood fibre insulation in between the studs of a timber frame and then 140mm of UdiSPEED and Diffutherm over the top of the studs. This will be externally rendered with the UdiPERL self-coloured render.The roof will be much the same make-up as the walls but with UdiTOP sarking board over the top of the joists.
To counteract the loss of thermal mass by not using Tradical Hemcrete we have opted to use 22mm clay boards on the ceilings throughout and also in all of the internal partition walls. The clay boards are good thermally but also extremely good acoustically, between 4 and 8 times better than a solid 100mm concrete block wall.
In a slight break from using our own products we are using Logixx Polystyrene ICF blocks (Insulated Concrete Formwork) to construct the walls of the basement and also the whole floor slab will be on 300mm of Isoquick Polystyrene insulation. Before anyone asks, No, we can’t do it on LECA or on ‘something more natural’.
Sadly, the existing bungalow on the site is not practical for us and so will be demolished shortly after Christmas when the main works start. Anyone wanting some exercise with a sledge hammer is more than welcome to contact us!!!!